(1) Field of the Invention
This invention relates to a zero-cross point detecting circuit. More particularly, the present invention relates to an improved zero-cross point detecting circuit in which, in order to detect a PG (a phase generator) signal for the purpose of detection of the rotation phase and rotation speed of a magnetic disk, the center of a PG yoke disposed in a central core of the magnetic disk is detected by a PG coil interlinked by leakage flux of a central-core attracting magnet provided in a spindle motor, so and to obtain the PG signal accurately indicative of the center of the PG yoke, as described in detail later.
(2) Description of the Prior Art
So-called magnetic disk cameras have recently been developed, which electronically record still pictures on a magnetic disk instead of a silver-halide photographic film. This magnetic disk camera comprises an integral combination of an image pick-up device such as a solid-state image pick-up element or an image pick-up tube and a magnetic disk type recording device. More precisely, as shown in FIG. 22 an optical image of an object 2 is picked up in the form of a still picture by an image pick-up unit 3 through a lens 1, and, after the raster scan type picture signal generated from the image pick-up unit 3 is subjected to signal processing including frequency modulation in a signal processing circuit 6, an output signal of the signal processing circuit 6 is applied through a recording amplifier 7 to a recording magnetic head 5 to be recorded on a predetermined circular track on a rotating magnetic disk 4. The magnetic disk 4 is rotated by a drive unit 8, and the position of the magnetic head 5 on the magnetic disk 4 is determined by a head positioning unit 9. A system for reproducing such a magnetically recorded still picture comprises an integral combination of a magnetic disk type reproducing device and a picture display unit such as a television set or a hard copy dispensing unit such as a printer. More precisely, as shown in FIG. 23, a predetermined track on a rotating magnetic disk 4 is repeately scanned by a reproducing magnetic head 10, and, after an output signal of the magnetic head 10 is applied through a reproducing amplifier 14 to a signal processing circuit 15 to be subject to signal processing including demodulation an output signal of the signal processing circuit 15 is applied to, for example, a television set 11 to display a visible picture of a soft copy on the display screen. Alternatively, the output signal of the signal processing circuit 15 is applied to a printer 12 to be printed out on a recording sheet 13 to provide a hard copy. The magnetic disk 4 is rotated by a drive unit 16, and the position of the magnetic head 10 on the disk 4 is determined by a head positioning unit 17.
By the way, for the purpose of practical use of the magnetic disk camera described above, it is desirable to standardize various particulars of the camera taking into consideration the compatibility of the apparatus. The particulars to be standardized include the shape and size of the magnetic disk 4, the magnetic characteristics of the magnetic disk 4, the rotation speed, track pitch and track width of the magnetic disk 4, the method of picture signal recording including modulation, the recording starting position of the vertical synchronizing signal in a picture signal on a track, and the reference point for determining the recording starting position of the vertical synchronizing signal. The desired standardization of such particulars have been virtually completed up to now. Further, for the purpose of facilitating handling of the magnetic disk 4, especially, for the purposes of, for example, facilitating mounting and detachment of the magnetic disk 4 in and from the body of the magnetic disk camera or reproducing apparatus, improving the accuracy of the mounted position of the magnetic disk 4 in the magnetic disk camera or reproducing apparatus and protecting the recording surface of the magnetic disk 4 against damage, packaging of the magnetic disk 4 has been attempted, and the particulars relevant to the packaging have also been standardized up to now.
As a result of the standardization of the various particulars described above, how to detect the rotation phase of the magnetic disk 4, hence the reference point for recording the vertical synchronizing signal, with high accuracy, has become an important problem. This problem will be explained with reference to FIGS. 24 to 29. FIG. 24 is a plan view of the package (commonly called a pack) of the magnetic disk 4, FIG. 25 is an enlarged sectional view taken along the line A-A in FIG. 24, FIG. 26 illustrates the format of recording, FIG. 27 shows the waveform of leakage flux interlinking a PG coil, FIG. 28 shows the waveform of the output voltage of the PG coil, and FIG. 29 illustrates a prior art method of PG signal detection.
As shown in FIGS. 24 and 25, the package generally designated by the reference numeral 18 is broadly divided into the magnetic disk 4, a central core 19, a PG yoke 20 provided for detecting the rotation phase of the magnetic disk 4, hence, the reference point for determining the recording starting position of the vertical synchronizing signal, and a jacket 21.
The magnetic disk 4 has a central opening and is fixed in position by being sandwiched between a core body 19c and a soft-magnetic plate 19d of the central core 19 extending into the central opening. The core body 19c of the central core 19 is made of a non-magnetic material. The core body 19c has a central bore 19b of generally pentagonal shape for receiving a spindle 24 therein, and a leaf spring 22 is mounted on one of the inner walls of the bore 19b to normally impart its spring force toward the center of the bore 19b. When the spindle 24 rotated by a motor 23 is inserted into the bore 19b of the central core 19, the spindle 24 is pressed against two inner walls of the bore 19b by the force of the leaf spring 22, so that the center of the spindle 24 aligns accurately with the center of the central core 19.
The jacket 21 is a flat hollow casing in which the magnetic disk 4 and the associated central core 19 are rotatably and vertically slightly movably received or accommodated. The jacket 21 includes an upper panel 21a and a lower panel 21c. The upper panel 21a is formed with an opening 21b through which the central core 19 is partly exposed and a window 21f through which a regulating plate (not shown) is disposed opposite to the magnetic disk 4. The lower panel 21c is formed with an opening 21d through which the spindle 24 is inserted and a window 21e through which the magnetic head 5 or 10 is brought into recording or reproducing contact with the magnetic disk 4. The windows 21e and 21f are provided with respective shutters 21g which are opened in the operation mode only. An annular guide 19e is provided on a lower surface of a plate 19d of soft-magnetic material to guide the rotation of the jacket 21. Further, the spindle 24 includes an annular flange 24a extending into the annular guide 19e of the central core 19, and an annular magnet 25 for attracting the soft-magnetic plate 19d of the central core 19 is disposed inside the annular flange 24a of the spindle 24.
The PG yoke 20 is in the form of a pin of a soft-magnetic material longitudinally buried at a predetermined position in the central core 19 to make magnetic contact with the soft-magnetic plate 19d. A PG coil 26 is mounted at a predetermined position in the body of the magnetic disk camera or reproducing apparatus to be magnetically coupled to the PG yoke 20. The PG coil 26 is electrically connected to a circuit 27 in which a PG signal required for controlling the rotation phase and rotation speed of the magnetic disk 4 is produced on the basis of an output voltage of the PG coil 26. The mechanism for detachably mounting the package 18 in the body of the magnetic disk camera or reproducing apparatus will not be described herein as it has not any concern with the present invention.
Referring to FIG. 26 showing the recording format according to, for example, the NTSC system, the recording starting position of the vertical synchronizing signal (V-sync) is so standardized that the leading edge 28 of the V-sync is delayed by an angle .THETA. from a reference position 29. This reference position 29 is determined to be the position lying on the line connecting the center of rotation of the magnetic disk 4, hence, the center of rotation of the central core 19 to the center of the PG yoke 20. It is also so standardized that, in the system where only one of two filed signals composing one frame of a picture signal is recorded on one track in the record mode, and a thru filed signal and a filed signal obtained by delaying the former signal by 1/2 of one horizontal scanning period (1H) or 0.5H are alternately changed over at a time interval of one vertical scanning period (1V) to reproduce a frame signal in the playback mode, a required switching point should coincide with the reference position 29 described above. In view of the requirements described above, it is necessary to detect the center of the PG yoke 20 by some means. In FIG. 26, the reference numerals 30 and 31 designate a picture signal and tracks respectively, and the reference symbols #1 to #10 designate the numbers of the hoizontal scanning period.
When the package 18 having the structure described above is mounted in the magnetic disk camera or reproducing apparatus, the central core 19 is force-fitted at its bore 19b on the spindle 24 while being automatically centered, and the soft-magnetic plate 19d of the central core 19 is magnetically attracted to the spindle 24 by the function of the magnet 25. Therefore, when the motor 23 is then driven to rotate the spindle 24, the central core 19 and the magnetic disk 4 fixed thereto rotate integrally with the spindle 24 under influence of the biasing force of the leaf spring 22 and the attracting force of the magnet 25. Also, since the PG yoke 20 buried in the central core 19 is magnetized by the magnet 25 through the soft-magnetic plate 19d, leakage flux of the PG yoke 20 interlinks the PG coil 26. Therefore, when the PG coil 26 is mounted at a predetermined position, a point 32 where the flux interlinking the PG coil 26 exhibits its maximum intensity as shown in FIG. 27 corresponds to the center of the PG yoke 20. In terms of the waveform of the output voltage of the PG coil 26, this point 32 exhibiting the maximum flux density or intensity indicates the zero-cross point designated by the reference numeral 33 in FIG. 28. Therefore, when the relative positions of the PG coil 26 and the magnetic head 5 (or 10) in the direction of rotation of the magnetic disk 4 (the direction shown by the arrow 34 in FIG. 26) are previously determined, the recording position of the leading edge 28 of the V-sync and the required switching position can be determined on the basis of the zero-cross point 33 of the waveform of the output voltage of the PG coil 26. For this purpose, it is necessary to obtain a signal or the so-called PG signal accurately indicating the zero-cross point 33.
According to a prior art practice, this PG signal is obtained by differentiating the output voltage waveform of the PG coil 26 by a differentiator, and, then, comparing the differential output signal 35 with a predetermined or fixed reference voltage 36, as shown is FIGS. 29(a) and 29(b). Consequently, it is inevitable that the leading edge 37a of the PG signal 37 deviates by a time .DELTA.t relative to the zero-cross point 33.
No problem arises when this time deviation .DELTA.t is constant. However, this time deviation .DELTA.t is generally dependent upon the level and shape of the differential output signal 35 and dependent also upon the level of the reference voltage 36. Especially, the level of the differential output signal 35 is dependent upon the level of the output voltage of the PG coil 26, and the level of the output voltage of the PG coil 26 is principally dependent upon the positional relation between the PG coil 26 and the PG yoke 20, in addition to the magnetic characteristics and shape of the PG yoke 20. In this case, the PG coil 26 is mounted in the body of the magnetic disk camera or reproducing apparatus, whereas the PG yoke 20 is fixedly mounted in the package 18 which is independent of and detachable from the body of the apparatus. Therefore, it is difficult to accurately maintain the desired fixed positional relation between the PG coil 26 and the PG yoke 20. Especially, in the package 18, an error occurs inevitably in the amount .DELTA.l of protrusion of the PG yoke 20 from the upper surface of the central core 19, and an error also occurs inevitably in the distance r between the center of the central core 19 and the center of the PG yoke 20. Thus, depending on different packages, the time deviation .DELTA.t of the leading edge 37a of the PG signal 37 from the zero-cross point 33 tends to greatly vary.
Therefore, it is necessary to develop a method according to which the zero-cross point 33 itself of the output voltage of the PG coil 26 corresponding to the center of the PG yoke 20 can be directly detected without accompanying any time deviation .DELTA.t.
In the case of a VTR (a video tape recorder), the method shown in FIGS. 29(a) and 29(b) is also employed for the detection of the PG signal. In the VTR, however, the PG yoke is mounted on the rotary head drum, and the positional relation between the PG coil and the PG yoke is fixed regardless of withdrawal and replacement of a magnetic tape. Therefore, the time deviation .DELTA.t is peculiar to each VTR and can be compensated as desired.